A typical electrolyte alkali metal halide cell is an enclosed container which is physically partitioned into at least two distinct regions by means of a permeable intermediate barrier or cell separator, such as a diaphragm or synthetic microporous separator. During the electrolysis of an alkali metal halide solution, hydrogen and alkali metal hydroxide are formed at the cathode, while chlorine and oxygen are formed at the anode. When the alkali metal is sodium, the electrolytic solution in the cathode compartment, i.e. the catholyte may contain approximately 12-17% NaOH, 15-20% NaCl and negligible, e.g. about 10 ppm NaOCl. However, when the current flow to the cell is interrupted, such as when a cell is subjected to routine maintenance during a shutdown period, the concentration of sodium hypochlorite in the catholyte cell increases significantly up to 2 gms/liter or more. Such concentrations of sodium hypochlorite can have an immediate adverse effect on the transition metals contained in the cathode, causing such metals to dissolve in the solution and ultimately leading to the failure of the cathode coating. In addition, these metals will also deposit within the diaphragm or membrane. These two events will cause a deterioration in cathode and membrane performance. That is, there is a loss of catalyst from the cathode or a higher hydrogen overvoltage and therefore a loss of cell voltage performance caused by the corrosion. In addition, there is a higher cell voltage and a lower current efficiency caused by the deposition cf the metal in the membrane or diaphragm.
In the start up of the multi-compartment cell there is generally a long period of time before the cell is operational. That is, during the period in which the cell is being charged with aqueous solution and there is no current flow, deposition of metals and corrosion occurs. The metals which are generally deposited are the transition metals.
The rapid increase in sodium hypochlorite concentration in the catholyte of a chlor alkali cell which occurs during periods of current interruption is caused by the convection flow and diffusion of hypochlorite ions from the anolyte. At the cathode, the sodium hypochlorite is reduced to sodium chloride and water as follows: EQU NaOCl+2H.sup.+ +2e.sup.- .fwdarw.NaCl+H.sub.2 O
The corresponding oxidation of the cathode transition metal, illustrated below by nickel, can be designated as follows: EQU Ni.fwdarw.Ni.sup.++ +2e.sup.-
The overall reaction can thus be designated as: EQU Ni+NaOCl+2H.sup.+ .fwdarw.Ni.sup.++ +NaCl+H.sub.2 O
The nickel in the cathode is thus ionized and becomes soluble in the catholyte causing dissolution of the coating.
Various coating materials have been suggested to improve the hydrogen overvoltage characteristics of electrolytic cell cathodes in an economically viable manner. A significant number of prior art coatings have included transition metals other than iron or steel, such as ruthenium, cobalt and nickel, or mixtures, alloys or intermetallic compounds of these metals with various other metals, and their oxides. Such metals during shutdown are deposited on the membrane or diaphragm.
The recovery of the metals in recent years has become important as a result of the high prices for catalyst materials and the increase in cost of such metals. It is further important as a cost factor to be able to reuse the membrane in cases where maintenance is performed before the normal operational life of the membrane has expired.
U.S. Pat. No. 4,055,476 discloses the continuous addition of nickel-based catalysts to an electrolytic diaphragm cell brine feed to prevent the formation of chlorates in the cell by decomposing sodium hypochlorite. Other reagents which are disclosed as being useful for this purpose include hydrochloric acid, sodium tetrasulfide, and various nickel and cobalt compounds. However, this patent does not recognize the utility or rejuvenating the diaphragm as a result of metal contamination during periods of current interruption or cell shutdown.
U.S. Pat. No. 4,680,098 to Yuehsiung discloses an electrodialysis process for extracting cobalt and manganese from catalysts recovered from a system for preparing trimellitic anhydride. Such a system can also be utilized for the extraction of cobalt and manganese from solutions obtained during the rejuvenation of the membranes in the present invention.
It is therefore an object of the invention to provide a method for extracting catalyst metals from the membrane of an electrochemical cell.
It is a further object of the invention to provide a process for the rejuvenation of a membrane separator of an electrochemical cell.
It is still further object of the invention to improve the performance of a chlor alkali cell after a shutdown period by the rejuvenation of its membrane separator.